Been to STX three times (once on a cruise, twice for a week's visit), and never heard of Kings beach; doesn't mean it doesn't exist, but make sure you have accurate info (if you can provide more info about location, I can probably confirm for you).
Buck Island (which is a completely different Buck Island from the one mentioned above that is near St. Thomas) is a boat ride from Christiansted, which is about a 30 minute drive from where the ships dock in Fredericksted. The cruiseline offering should include the transport to C'sted. You must take a licensed provider to go to BI. No ferries, no facilities. It is a national park, is absolutely fantastic, well worth the time and money. Water clear as gin, and the most fantastic turquoise color! *sigh*
That said, the first time we went, we took a cruiseline offering, and while it was a good way to do it for a first visit, we didn't really enjoy the experience--too far to go in too short a time, and too many people on the boat, etc. A real "moo" experience. Ya know, like cattle.
Went again on our own on the Diva (mentioned above) a few months later, and had a fantastic time--beautiful boat, lovely sail out, snorkel at the reef, then lunch and a 45 minute stop on the beach. But this is a full day excursion--10-4 or so. The beach there is breathtaking, one of the prettiest in the Caribbean, IMHO. Be advised, however, that often the cruiseline trips don't even stop at the beach, so ask. They have time only to do the snorkeling, which is done directly from the boat. The problem is BI is fairly far, and you may not have choice of providers given port timing... If you check out the Diva, make sure they can accommodate your arrival/departure times. Also, it leaves from Green Cay, which is about 5 minutes east of Christiansted. I was also there last month, and went with Tommy from Mile Mark on a small catamaran--just 6 of us--sailed from the downtown C'sted harbor. Also a great experience, but not sure you'll have time on a port stop to do this one either.
If you want something that isn't so time consuming, go to the beach at Changes in L'Atitude Bar just north of Frederiksted--not even a mile from the ship. The bar has friendly staff and customers, beach chairs, umbrellas, clean bathrooms, etc., and snorkeling is decent directly from shore (though definitely not as good as Buck Island). You only need to buy a drink to use their facilities.
Cane Bay Beach on the North Shore Road, which requires a 20 minute cab ride from the port, also has decent and easy snorkeling directly from shore--swim out about 40 feet over the shallows, and swim parallel to shore through the reef. Water is very shallow. Be aware of strong currents at far east end though. Shoys Beach, near the Buccanneer Hotel, is gorgeous, very quiet, and perfect for swimming/sunning, but not for snorkeling--no fish there. Cabs are expensive--so share the ride if you can. Cab prices are set by law, and are published in the pink St. Croix This Week which you can pick up almost anywhere.
btw: if you want to shop, go to Christiansted. Frederiksted is a strange place, and not the place by which to judge the rest of the island.
Happy to answer other STX questions. I know the island fairly well. Write to me here. If you are interested in more about the island (and other beaches), read a trip report I posted at http://caribtravelnews.com/c0700_03.htm#stcpeg
Hope this helps--sorry so long.
Have a great time!